In known processes of the type contemplated in the present invention, granular material is admitted into a tube mill and is ground and passed through different compartments. After passing through the tube mill the material is discharged from the mill. The grinding in the final compartment takes place with the assistance of grinding bodies having an average piece weight between 20 and 40 grams (g). The minimum size is typically about 20 millimeters (mm). As a result of the free flow area required together with the strength and manufacturing requirements, small grinding bodies are not used since the slots in conventional outlet diaphragms used in the final compartment cannot be constructed sufficiently narrow so as to allow the use of smaller grinding bodies and ensure effective screening of the ground material.
Although it has been widely recognized that in order to achieve optimum grinding economy, the size of grinding bodies used in the final grinding compartment of a mill should be far smaller than that presently in use, up until the present no method or apparatus has been devised in which such smaller grinding bodies may be used.
We have invented a grinding method and apparatus according to which optimum grinding economy is achieved in a tube mill having two or more compartments. According to a significant feature of our invention, the tube mill utilizes grinding bodies which are particularly dimensioned in accordance with the size of the particles of materials required in the final product.